Hi Shahzad,
I fell Michael's answer is perfect as it truely works out in our case. I mean, when we (in our projects) leave the pin unconnected, it (8051 derivative) reads it as HIGH unless some external connection makes it low. Now, if you want the inverse of it, simply use a low-value pull-down resistor and that should work out.
Erik, he wants to see if the pin is LOW (which would indicate to him if it is floating), so I think what Michael said is correct.

-Harsha